Although different types of energy storage devices are made by different electrodes, electrolytes and other materials, thus with different energy storage mechanisms, the basic constitutions and the fundamentals of the devices are more or less the same. One example is that every type of energy storage device, including ultracapacitors, lithium ion capacitors, batteries, and fuel cells and hybrid cells which are the combination of the above devices, need electrodes. Another example is that, in all of these devices, if an electrode is made thicker, it will store more energy but with less power. Another example is that, among all the critical components, the electrode is one of the most important components in the energy storage devices. It plays a major role in determining the performance, the reliability or the field life, as well as the cost of the devices.
Energy density and power density are the most important performance characteristics for energy storage devices. In order to have high energy densities, electrodes need to be made with the highest amount of active materials per surface area or per volume. In the example of an ultracapacitor electrode, the more activated carbon is packed in the electrode, the higher capacitance the device will have, the higher energy density the device will provide. Therefore, a higher density electrode, i.e., with more active materials, is preferred. A higher density electrode also promotes more direct contact and a greater number of contacts between the active material particles, the conductive material and active material particles, thus promoting the potential for greater electrical passes inside the electrode, and thereby improving the electrical conductivity and the power density of the electrode. On the other hand, non-functional materials, such as binders and conductive carbon, do not add any capacitance or the energy to the electrode, thus should be used as minimally as possible. Due to the non-conductive nature of a binder, excessive usage of binders in the electrode will cause high electrical resistance in the device, thus lower the power density. Therefore, improving electrode packing density and reducing the non-functional material usage, are the most important ways to improve energy and power density of the electrodes.
Ultracapacitors store electrostatic energy in an electrode/electrolyte interface layer. At the electrode and electrolyte interface, a layer of ions is formed to balance the electrical charge on the electrode. The charge and discharge process is a pure physical process, there is no chemical reaction associated with the process and more importantly, to alter or degrade the materials. Therefore, the life of the devices in the applications is theoretically to be forever. However, in reality, there are always some undesired impurities in the device that cause chemical reactions during application, thus harming the device reliability and shortening the field life. The typical impurity in an Ultracapacitor is residue solvent in the electrode, or the impurities in the raw materials, especially in the activated carbon due to the excellent absorbing ability of the activated carbon.
Cost of an electrode is determined by the combination of the cost of the materials and the cost of manufacture. Less usage and cheaper raw materials, fewer manufacturing steps, less energy used in the manufacture process, high production through put and a higher yield of the manufacture are the main means to achieve a low cost energy storage product.
To make an electrode by a coating process, a solvent or an aqueous solution or both are used to dissolve binders, followed by mixing the binder solutions with other powder materials to form a slurry. The most widely used solvent in the battery, lithium ion capacitor, Ultracapacitor and hybrid cell electrode manufacture process is N-methylpyrrolidine, also known as NMP. The typical percentage of solids in the slurry is 15-20%. And the typical binder content in the total powder content is 3-6% and/or up to 15% to get the strong electrodes. The slurry is then coated onto a treated or non-treated current collector by a cylindrical roller. The current collector along with the coating layer is passed through a long dryer, where the solvent is dried and removed from the electrode.
The problems associated with the coated electrodes are: Problem #1—since the binder is dissolved in a solution and it flows into active materials to block the active material surfaces, this reduces the active materials functionality and also increases the device's overall resistivity, thus resulting in reduced device energy and power density; and Problem #2—there is always a residue from the solvent in the coated electrode due to large amount of solvent soaked in the electrode materials during the manufacturing process. Solvent such as NMP is very difficult to dry and remove. Therefore, an electrode made by a conventional coating method does not provide a long life energy storage device.
Other related problems include: Problem #3—solvent added to the electrode formulation needs to be removed. And, there is large amount of energy needed to dry and remove the solvent, which adds an additional cost to the manufacture of the product.
Finally, yet another problem that exists is: Problem #4—since the binder used is dissolvable in solvents by nature, the binder will be dissolved into the electrolyte chemically or electrochemically sooner or later, the particles in the electrode eventually lose contact to each other or to the current collector, and this inevitably leads to early energy storage device failure.
To make an electrode by an extrusion process, a solvent, normally with high lubricating quality, is added to a powder mixture, followed by an extensive mixing using a screw extruder or some other extruder. During the mixing and forward pushing process, large shear force is applied to the powder mixture, where the binder is fibrilized and acts to connect the other particles together. The typical percentage range of solids in the extrusion electrode formulation is 40%-60%. And the typical binder content in the total powder content is 10-15%. The extruder forced well mixed materials out from the exit and an electrode film is formed by a calendar or multiple calendar stations to the required thickness. The film passes through a long dryer to dry and remove the solvent.
The extrusion process for making an electrode does not have problems #1 where the binder flows into active materials to block the active material surfaces as much, and Problem #4 where binder will be dissolved into the electrolyte chemically or electrochemically sooner or later, as listed and outlined above in coating process, but maintains the problems associated with Problem #2 where there is always a residue from the solvent, since the lubricating solvents are normally very difficult to be dried out, and Problem #3 that a large amount of energy is needed to dry out the added solvent. In addition, new problems occur in the extrusion process. Problem #5 a high capital investment is required. The extrusion equipment along with the calendars followed by long dry ovens all are very expensive investments and the manufacture process is very complicated, therefore, the overall cost to make an extruded electrode is very high.
Another new problem, problem #6, that is, a higher percentage of the binder has to be used to make a strong enough electrode film which associates two new problems: Problem 6A—less active materials can be added to the electrode formulation, thus lower energy density of the device; and Problem 6B—this larger amount of binder blocks the surface area of the active materials, further lower the energy density of the device, and Problem 6C—this larger amount of binder blocks the electrical flow between the active material particles, thus increased resistance and reduced power density for the device.
To make an electrode by a dry process, powders were dry mixed and subjected to an extensive mixing, where the binder is fibrilized and is formed into a matrix to support the other particles to form an electrode film. The dry process solved all four problems associated with the coating process, and the problem #5 in the extrusion process however, it carried over extrusion process's problem #6, which is it needs a large quantity of the binder to support and to make a strong enough electrode film with adequate film density. The exact reason why it needs large amount of binder in the formulation is unknown. However, it is believed by the current inventor that the binder used in the dry process without activation by mixing with certain solvent provides less adhesion and therefore is less effective.
In this respect, before explaining at least one embodiment of the invention in detail it is to be understood that the invention is not limited in its application to the details of construction and to the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments and of being practiced and carried out in various ways. In addition, it is to be understood that the phraseology and terminology employed herein are for the purpose of description and should not be regarded as limiting.